Several groups of mice will be exposed to repeated shifts of their daily lighting schedule--a strong synchronizer of their circadian rhythms--beginning at different ages (young adult, middle age or old age) and continuing for the rest of their lives. Other groups will experience effects of two strong circadian synchronizers, lighting and feeding, opposing or reinforcing each other in repeated shifts. Lifespan will be the major endpoint used to test any adverse or perhaps even beneficial effects of such schedule shifts, simulating prolonged shift-work in human beings. The response of circadian rhythms to these repeated shifts also will be examined to possibly shed some light on mechanisms underlying any effects on lifespan. The incidental recent discovery in our laboratory of changes in telemetered core temperature rhythm with aging of stroke-prone rats (findings suggesting that the suprachiasmatic nucleus might constitute a mechanism of aging) also will be pursued along with concurrent opportunistic studies, checking on the extent to which the same findings may apply to human beings, with particular reference to shift-work.